An Analysis of the Lubrication between the Piston Rings and Cylinder Wall of a Running Engine

Abstract

A method has been developed for the study of the lubrication in the cylinder of an engine under actual running conditions by an analysis of the electrical conductance between a piston ring and the cylinder wall. One of the compression rings was electrically insulated from the piston and a current passed from this ring to the cylinder wall: the potential drop between the moving surfaces was calibrated in terms of the resistance. With good conditions of lubrication, the electrical resistance is high, while low resistance values indicate breakdown of the lubricant film and consequent abrasive wear of the surfaces. A study of this resistance variation was made by means of a cathode ray oscillograph with the time base synchronized with the stroke of the engine. Results indicate that, with the oils used, the surfaces of the piston ring and the cylinder wall are not separated by a continuous oil film throughout the entire cycle. Intermittent breakdown of the oil film occurs to a varying degree under all circumstances, even using oils much heavier than are common in automobile practice. Also, in general, the resistance is lower; that is, more metallic contact occurs in the regions of top and bottom dead centres than during the remainder of the cycle. The method confirms the improvement in lubrication expected with increase of engine speed, or with increase of viscosity. The effects of gas pressure behind the rings are shown to be small, at least with the low brake mean effective pressure in the particular engine studied. A marked deterioration in the quality of the lubrication accompanies any increase in the temperature of the oil film on the cylinder wall—greater deterioration than can be accounted for simply by the decrease of viscosity due to the temperature rise. Finally, the results demonstrate the applicability of this method of analysis to problems of cylinder lubrication and abrasive wear, its chief value being the rapidity with which results may be obtained as compared with conventional wear tests.